An optical parametric amplifier (OPA) is an optical amplifier based on the nonlinear optical effect known as 3-wave mixing. In the 3-wave mixing interaction, an incident pump photon is annihilated and two lower-energy photons (signal and idler) are created in the presence of a nonlinear optical crystal. The vacuum wavelengths of the signal and idler photons are constrained such that conservation of energy is satisfied, i.e. the sum of the signal and idler photon energies is equal to the pump photon energy. In order to efficiently transfer energy from the pump to the signal and idler, the photons must meet a phase matching condition (momentum conservation). In one type of phase matching, called birefringent phase matching, the phase matching conditions can be influenced by adjusting the orientation of the nonlinear crystal with respect to the incident pump direction (angle tuning). In certain types of birefringent phase matching, called Type II phase matching, the idler polarization is orthogonal to both the pump and the signal polarizations. Other types of phase matching are also possible, such as quasi phase matching.
A typical OPA is implemented as follows. A strong pump beam of a first wavelength is mixed with a relatively weak signal (seed) beam having the desired output wavelength. The signal beam is amplified and an idler beam is created by depleting the pump beam, as discussed above. In this manner, the OPA produces a strong output signal beam at the same wavelength as the signal seed and simultaneously reduces the power of the pump beam. Thus, the OPA is considered to “convert” the pump beam into the desired signal.
The process described above will be referred to as forward-conversion, i.e. power flows from the pump beam to the signal and idler beams. Under certain conditions, a process called back-conversion can occur where power flows from the signal and idler beams back to the pump beam. Back-conversion reduces and limits the pump-to-signal conversion efficiency in an OPA system. In addition, the signal beam quality can be degraded due to phase distortions resulting from back-conversion.
Back-conversion can occur when a phase-mismatch develops between the three waves. When multiple OPA stages are employed, it is often desired to remove the idler beam between stages since it is difficult to keep all three waves properly phased when propagating from one stage to the next. Removing the idler beam guarantees power flow from the pump to the signal beam in the subsequent OPA stage, regardless of relative phase between the three waves. Back-conversion can also occur within an OPA crystal stage when the pump photons have been completely depleted. If signal and idler photons are still present, then power flows back to the pump beam. The latter back-conversion process can occur in one or more regions of the pump beam transverse profile, if it has a non-uniform spatial profile.
Conventional methods for idler extraction include the use of dichroic mirrors that transmit the signal and pump beams and reflect the idler beam. Dichroic mirror coatings with reflectance specifications at 3 wavelengths typically include a large number of dielectric film layers and can have a low laser damage threshold. This is problematic in high-peak-power or high-average-power OPA or optical parametric oscillator (OPO) systems. Other conventional methods include employing an optical medium between OPA stages that absorbs the idler power and transmits both the signal and pump waves. This approach is problematic in high-average-power OPA systems because the absorbed idler causes thermo-optical variations in the absorbing medium that can distort the phase of the pump and signal beams.